Acute promyelocytic leukemia cells (APL; FAB M3) have a specific balanced translocation t(15;17) (q22;12-21) that rearranges the retinoic acid receptor alpha (RAR alpha) and the promyelocytic leukemia (PML) genes. Two features distinguish APL from other leukemias. These are: (1) patient-derived APL cells express the normal RAR alpha and PML alleles and PML/RAR alpha, resulting from this translocation and (2) treatment of APL patients with all-transretinoic acid (RA) induces complete clinical remissions. Paradoxically, these responses are linked to expression of PML/RAR alpha, a rearranged retinoid receptor. Two attractive but not mutually exclusive hypotheses exist to explain these clinical responses of RA in APL. PML/RAR alpha exhibits a "dominant negative" effect on RAR alpha, or it antagonizes PML function. This NIH competitive renewal application seeks to better understand the paradoxic RA response of APL. This will be accomplished through studies with transgenic mice expression PML/RAR alpha, which were engineered during the prior funding period, and with the NB4 cell line. This is the unique APL cell line containing the t(15;17) and responding to the cytodifferentiation signals of RA. The specific aims are: (1) to engineer hammerhead ribozymes to preferentially cleave PML/RAR alpha and transfect these into NB4 cells to evaluate their growth and differentiation effects before and after RA-treatment of these APL cells ; (2) to determine whether the observed growth and differentiation effects are mediated through targeting of PML/RAR alpha and; (3) to identify pathways through which PML/RAR alpha signals its biologic effects in transgenic mice expressing PML/RAR alpha and in NB4 APL cells transfected with PML/RAR alpha cleaving hammerhead ribozymes. In this proposal, cellular, biochemical, and molecular genetic techniques are used to investigate the mechanism of action of RA, a new anti-cancer agent within a preclinical experimental model of APL. Through the stated aims we hope to better understand the role of PML/RAR alpha in the RA- mediated differentiation therapy of APL. These findings are relevant to the treatment of other human malignancies currently responsive or refractory to retinoid-based therapy.